A few days ago I wrote about localization , where waves in a medium can become trapped due to interference by scattering off disorder.  This is an extremely general phenomenon that applies to light, sound, and electronic waves in solids. Now I want to write about a phenomenon that is specific to electrons (or at least wavepackets that carry electronic charge, if we want to be very general).  Rather than the completely general arguments about conductivity scaling, now we are going to consider par