No, I don't use A/B tests for usability as they aren't very helpful to find usability problems.

A/B testing is more for conversion optimization, it's to measure how persuasive something is.

User testing is my preferred method (in it's simplest form just observing people doing a task), if that's not an option I like heuristic evaluations.

I suggest you actually look into what usability is about, you might enjoy it. Good luck with your assumptions.