Can I downgrade from macOS Sierra to Snow Leopard (OS X 10.6)? I have a 2016 MacBook.

First, OS X El Capitan was installed. Now, I've installed Sierra.

Can I downgrade to Snow Leopard?
If I can, is USB method okay?
You cannot downgrade the 2016 MacBook Retina to OS X Snow Leopard.
Apple does not support running OS version older than the one shipped with the hardware. Refer to the article Use the Mac operating system that came with your Mac, or a compatible newer version:

Your Mac is designed to work only with compatible macOS versions and builds. If you try to use an incompatible version or build, your Mac might behave unexpectedly.
Avoid using a version of the Mac operating system that is older than what came with your Mac.