PARIS — Alberto Contador won the Tour de France for a second time Wednesday, and Lance Armstrong capped his return to the race with an impressive third-place finish. Mark Cavendish of Britain collected his sixth stage win of this year's Tour in a sprint after the 101.9-mile course ride from Montereau-Fault-Yonne to the Champs-Elysees. Over nearly 3,500 kilometers and 21 stages of races over three weeks, Contador repelled many challenges in the mountains, excelled in the two time-trials – winning a pivotal race against the clock in the 18th stage – and won the first Alpine stage. Contador, the 2007 champion, also had to battle a rearguard action within his Astana team, where the comeback of Armstrong to the Tour after 3 1/2 years of retirement raised questions about who would be the team leader. Contador began the Tour on July 4 as the pre-race favorite.