I have generally heard what Hrotha says (good post BTW), but wish to clarify something I saw:
I had the fortune of watching the live feed of nearly the full stage (sans commercials) in 2004 to Plateau de Beille. This is where Mayo, who had previously crashed on stage 4 I believe, crashed again. Further up the road on the penultimate climb with a couple domestiques around, he stopped, now some 5+ minutes back of the leaders or so, and got off the bike in obvious pain. He stood and looked around for a few moments like he was done, a couple of riders rode by as he waited. He then got back on the bike and kept going. This was a pretty cool thing to see and he got a cheer out of both the room I was in, and from the fans gathered around on the side of the road. He made it all the way to the end of the stage, even though he was now well over 10 minutes back. Unfortunately he was unable to continue the next day. So whenever anyone describes him as being mentally weak or too emotional (not that anyone directly said that), this day comes to mind.
As to his B sample, while I'm convinced he was on EPO like everyone else, that there is the chance that, like what happened with Heras, his name ended up on the proverbial blacklist, and once one test was close enough, that was it for his career.
Really like to revisit those old days, reading old threads about it gives me maybe even more than rewatching the races.
That 2003 Stage to ADH was certainly something else, I got into cycling 1999, LAs first year of dominance, from the start I disliked him, was rooting for Zülle the first year, later for Ulrich and anyone who challenged him.
So by 2003 LA had won the TdF 4 times and each and every year made he pretty much decided the Tour in his favor already on the first major mountain stage, raising out of his saddle, attacking and putting 1 to 2 mins between him and the best of the rest. This continued for the rest of the mountains (yes he had a bad day in 1999 and 2000, but near the end of the tour and he was leading comfortably by then and could afford it, never being in real danger to lose the Tour). In 2001 and 2002 he was at his peak IMO, setting attack after attack and at the worst finish with his best competitors. Then 2003 and the ADP stage came along a for the first time it looked different, going into the climb and with USP accelerating early I expected nothing less than another attack with him easily winning the Stage (and pretty much the entire race ) but then his team was spent and that attack didn't come, he was riding along with Vino and Hamilton. Normally being isolated, he would leave those two behind and they would see him at the finish line. This time he wasn't better than the best after 5,6 GC contenders, that already was smth completely new. Sometime later Beloki, Mayo and a few others made it back into the first group and they started attacking, Beloki and Hamilton didn't get any real separation, but then Mayo did, a bit later also Vino. That was the first time I have seen some riders climbing genuinely better than LA. I believe LA wasn't really expecting it, otherwise he wouldn't have spent his entire team so early in the climb, they were clearly going all in, he was thinking to pull off the exact same thing he did in ADH 2001, when he went on the attack early and put 2 mins between him and Ulrich.
Later that Tour, when he won in Luz Ardiden, was the last time I have seen LA pulling off one of those attacks. The following years he was always staying with the best in the Mountains, while never really attacking before the finish line, relying on his superior TT- and TTT-ability of his teams.