  Scattering by slowly fluctuating degrees of freedom can cause a transient
localization of the current-carrying electrons in metals, driving the system
away from normal metallic behavior. We illustrate and characterize this general
phenomenon by studying how signatures of localization emerge in the optical
conductivity. The buildup of quantum localization corrections manifests itself
in the emergence of a displaced Drude peak (DDP), whose existence strongly
alters the low frequency optical absorption and suppresses the d.c.
conductivity. We find that, when compared to the well-studied low concentration
limit, many-body effects considerably enhance transient localization and the
ensuing DDP phenomenon, due to the back-reaction of the charge density on the
bosonic field. Our results indicate that important quantum corrections to
transport should be present in bad metals with slowly fluctuating degrees of
freedom.
