If the ball goes behind the batsman, and then goes over the wickets without touching the wickets, then is it a wide ball?
No, this would not a wide ball. A wide is defined by Law 25:
If the bowler bowls a ball [...] the umpire shall adjudge it a Wide if [...] in his opinion the ball passes wide of the striker where he is and which also would have passed wide of him standing in a normal guard position.
(my emphasis). A striker standing in a normal guard position would be able to play a ball which passes over the stumps, so this is not a wide.