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

We report a specially designed transverse magnetic focusing experiment on a two-dimensional electron system confined to a square quantum well with two electric subbands occupied. The reflecting barrier of a usual magnetic focusing device is replaced by a surface-gate-induced, tunable potential barrier that allows us to selectively reflect and magnetically focus the upper subband electrons while the lower subband electrons pass over the barrier and are not focused. We observe the focusing of the upper subband electrons. The weakness of the focusing signal for the upper subband ballistic electrons suggests that they have a surprisingly short ballistic mean free path compared to the lower subband electrons.