Ballistic transport in the upper subband of a two-dimensional electron system

We report the observation of ballistic transport of upper subband electrons in a two-dimensional electron system (2DES) confined to a square quantum well with two occupied electrical subbands. We have performed two types of experiments. In the first type, we use a specially designed transverse magnetic focusing device in which the usual reflecting barrier is replaced by a surface-gate-induced, tunable potential barrier to selectively reflect and magnetically focus the upper subband electrons. In the second type of experiment, we create a surface-gate-induced, periodic potential in a double-subband-occupied 2DES and measure the magnetoresistance along the modulation direction. Fourier analysis is used to separate the commensurability oscillations originating from ballistic electrons of different subbands.