Nonadiabatic transport through quantum dots

Nonadiabatic transport through a constriction in a quasi-one-dimensional quantum wire or through a quantum dot connected to quantum-wire leads occurs when different lateral modes are coupled by scattering off changes in the geometry of the conducting channel. We present calculations of coherent resonant tunneling through quantum dots that are connected nonadiabatically to the quantum-wire leads either by abrupt junctions or by smoothly tapered junctions. We determine how the nonadiabatic transport is influenced by the type of transport through the dot by studying transmission through dots that are connected to the leads across potential barriers with different heights (ballistic transmission for no barriers, resonant tunneling for large barriers). Nonadiabatic effects are enhanced when the transport through the dot occurs by resonant tunneling rather than ballistic transport. Resonant transport through dots with smoothly tapered connections can be nonadiabatic even when ballistic transport through constrictions with the same connections is adiabatic. These results justify the recent suggestion that the mode mixing that occurs during resonant tunneling through a vertical quantum dot is strong enough to open the additional transport channels needed to understand the fine structure observed in the resonant tunneling.