Were Back Scabbards Used?

Scabbards were historically, albeit rarely, worn across the back, but only by a handful of Celtic tribes, and only with very short lengths of sword.

Did people carry swords on back?

What is the evidence for medieval warriors wearing their sword on their back? Little to none. There is some suggestion that the Scots did it, and it seems reasonable that other large, two handed swords may have been transported that way. But there is little to suggest that a sword was worn on the back.

Are back sheaths good?

And back scabbards for swords are most definitely not a reasonably good idea. There is only one good reason to put a weapon on your back, which is to get it out of your way. Swords were sometimes carried this way, and it was a signal that you didn't expect to use your sword.