Mode spectroscopy and level coupling in ballistic electron waveguides

A tunable quantum point contact with modes occupied in both transverse directions is studied by magnetotransport experiments. We find conductance quantization that can be suppressed by degeneracies of one-dimensional modes. The mode spectrum is determined as a function of the magnetic field of different orientations with respect to the quantum wire. A magnetic field applied 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 observed as a function of the parallel magnetic field, but not for zero field or perpendicular field directions, indicating coupling of the subbands due to nonparabolicity in the electrical confinement.