OBSERVATION OF BALLISTIC TRANSPORT IN THE UPPER SUBBAND OF A 2-DIMENSIONAL ELECTRON-SYSTEM

We report a specially designed transverse magnetic focusing experiment on a two-dimensional electron system confined to a square quantum wel l with two electric subbands occupied. The reflecting barrier of a usu al magnetic focusing device is replaced by a surface-gate-induced, tun able potential barrier that allows us to selectively reflect and magne tically focus the upper subband electrons while the lower subband elec trons pass over the barrier and are not focused. We observe the focusi ng of the upper subband electrons. The weakness of the focusing signal for the upper subband ballistic electrons suggests that they have a s urprisingly short ballistic mean free path compared to the lower subba nd electrons.