Experimental measurement of scattering coefficients in mesoscopic conductors

We describe a general experimental approach yielding the entire transmission matrix of a multiprobe mesoscopic conductor. Results are presented for several new investigations with ballistic semiconductor nanostructures enabled by this technique. We measure the transmission coefficients for an open cross junction employing a sample design which is an almost literal realization of the Landauer-Büttiker model and verify that these transmission coefficients are consistent with Hall and bend resistance anomalies obtained by resistance measurements at low magnetic fields. Other investigations utilize a pinched cross junction in which two of the probes are separated from the channel by quantum point contacts. This series arrangement of point contacts allows a sensitive momentum spectroscopy of the emitted distribution. Even in the case of a single propagating mode, through the constrictions we observe modal features in the outgoing distribution. This indicates that the potential is spatially nonadiabatic even though conductance quantization is observed. This pinched geometry also allows the first fully characterized realization of weakly coupled probes.