  A field theoretical method is developed which permits us to study the
dynamics of vortices in disordered environments. In particular, we obtain a
self-consistent system of equations for disorder averaged quantities. Making
use of a Hartree-type approximation, we calculate the current-voltage (I-V)
characteristics in the flux flow regime. In order to probe incipient melting of
the vortex lattice we propose an experiment where interference steps in the I-V
characteristics are observed, which arise when a small ac field is superimposed
on the constant voltage.
