Mode spectroscopy and level coupling in ballistic electron waveguides

A tunable quantum point contact with modes occupied in both transverse dire ctions is studied by magnetotransport experiments. We find conductance quan tization that can be suppressed by degeneracies of one-dimensional modes. T he mode spectrum is determined as a function of the magnetic field of diffe rent orientations with respect to the quantum wire. A magnetic field applie d parallel to the direction of the current flow couples the modes. This can be described by an extension of the Darwin-Fock model. Anticrossings are o bserved as a function of the parallel magnetic field, but not for zero fiel d or perpendicular field directions, indicating coupling of the subbands du e to nonparabolicity in the electrical confinement. [S0163-1829(99)15235-4] .