  We report a crucial experimental test of the present models of the peak
effect in weakly disordered type-II superconductors. Our results favor the
scenario in which the peak effect arises from a crossover between the Larkin
pinning length and a rapidly falling elastic length in a vortex phase populated
with thermally excited topological defects. A thickness dependence study of the
onset of the peak effect at varying driving currents suggests that both screw
and edge dislocations are involved in the vortex lattice disordering. The
driven dynamics in 3D samples are drastically different from those in 2D
samples. We suggest that this may be a consequence of the absence of a Peierls
potential for screw dislocations in a vortex line lattice.
