We develop a novel self-consistent Green's function approach to study the d
ynamics of electronic transport across a metal/conjugated-oligomer/metal st
ructure. We find a crossover behaviour in transport from free electron to p
olaron-like, with increasing phonon frequency of the oligomer. In the forme
r. lattice motion lags behind the wavepacket of an incoming electron; where
as, in the latter, the lattice can follow the electron. To simulate lattice
fluctuations we study a pre-existing lattice distortion and find enhanced
sub-gap transmission. These results have implications for transport experim
ents on polymer light-emitting diodes and molecular wires.