Comment by klempner

The Landauer limit defines minimum energy for a bit *erasure*.

A reversible gate doesn't involve any such erasure and therefore Landauer's principle doesn't apply to it.

What will happen in practice if you do an entirely reversible computation is that you end up with the data you care about and a giant pile of scratch memory that you're going to need to zero out if you ever want to reuse it. Or perhaps you rewind the computation all the way back to the beginning to unscratch the scratch memory but you're going to at least need to pay to copy the output somewhere.