Vertical-mode dependence of coupling between an electron waveguide and reservoirs with two occupied subbands

Quantum point contacts (QPCs) have been fabricated by atomic force microscope lithography from a GaAs/AlGaAs heterostructure containing a two-dimensional (2D) electron gas with two subbands occupied. The QPC energy spectrum is characterized by two quantum numbers, for the vertical (s) and lateral (n) confinement. The one-dimensional modes were identified by using persistent recharging of the heterostructure supply layer while cooling the QPCs from room temperature to View the MathML source under a forward gate bias voltage. A stronger smearing of the conductance steps originating from the second vertical mode (s=2) compared to those of the ground mode (s=1) is observed, which we explain by an enhanced scattering at the coupling of the second vertical mode to the 2D reservoirs.