Imaging diffraction-limited electronic collimation from a non-equilibrium one-dimensional ballistic constriction

We report on the use of a low-temperature scanning probe microscope to inve stigate non-equilibrium electronic transport through a one-dimensional ball istic constriction. Trans-conductance images of electrons backscattered in the adjoining two-dimensional reservoirs show a weak acceptance cone consis tent with semiclassical collimation. The images also show a strong highly c ollimated beam of quasi-particles injected into each reservoir with a diver gence consistent with quantum mechanical diffraction. In the lower-chemical -potential reservoir these quasi-particles are interpreted as hot electrons while in the higher-chemical-potential reservoir they are interpreted as c onduction-band holes.