Heisenberg and the Bomb

Quite a good discussion here of the question of why Germany did not get far in its development of an atomic bomb in World War 2.   In particular, did Heisenberg and his German physicist buddies deliberately prevent it by telling Speer that (to paraphrase), sure, they'd love to do it, but it's just too big a job for Germany.  (Mind you, considering the incredible scale of the Manhattan Project, perhaps this was no great stretch of the truth at the time.)