Suppression of Shubnikov–de Haas resistance oscillations due to selective population or detection of Landau levels: Absence of inter-Landau-level scattering on macroscopic length scales

The Shubnikov--de Haas resistance oscillations of a wide two-dimensional electron gas are suppressed dramatically when current is injected with a quantum point contact which does not populate the upper Landau level. A similar suppression is observed when a quantum point contact is used as a voltage probe which does not detect the upper Landau level. The results are explained with a model in which the Shubnikov--de Haas oscillations arise from backscattering of electrons in the upper Landau level. It is shown that quantum ballistic transport in high magnetic fields can occur on length scales larger than 200 \ensuremath{\mu}m.