  A relation derived from the Kubo formula shows that optical conductivity
measurements below the gap frequency in doped semiconductors can be used to
probe directly the time-dependent quantum dynamics of charge carriers. This
allows to extract fundamental quantities such as the elastic and inelastic
scattering rates, as well as the localization length in disordered systems.
When applied to crystalline organic semiconductors, an incipient electron
localization caused by large dynamical lattice disorder is unveiled, implying a
breakdown of semiclassical transport.
