  We consider transport through finite quantum systems such as quantum
barriers, wells, dots or junctions, coupled to local vibrational modes in the
quantal regime. As a generic model we study the Holstein-Hubbard Hamiltonian
with site-dependent potentials and interactions. Depending on the barrier
height to electron-phonon coupling strength ratio and the phonon frequency we
find distinct opposed behaviors: Vibration-mediated tunneling or intrinsic
localization of (bi)polarons. These regimes are strongly manifested in the
density correlations, mobility, and optical response calculated by exact
numerical techniques.
