Prof. Robert M. Westervelt
Division of Engineering and Applied Sciences
and Department of Physics
Harvard University
Cambridge, MA  02138

Scanning a charged tip above the two dimensional electron gas inside
a GaAs/AlGaAs nanostructure images the coherent electron flow from
a quantum point contact at liquid He temperatures.  As the width of the
quantum point contact (QPC) is increased, its electrical conductance
increases in quantized steps of 2e^2/h.  The angular dependence of the
electron flow on each step agrees with theory, forming broad lobes at
submicron distances from the QPC, and fringes separated by half the
electron wavelength are observed.  At distances above 1 micron, the
electron flow forms narrow channels as a consequence of small angle
scattering, in agreement with recent theory for a QPC in a high mobility
two dimensional electron gas.