Nonlinear transport and noise thermometry in quasiclassical ballistic point contacts

We study nonlinear transport and nonequilibrium current noise in quasiclassical point contacts (PCs) defined in a low-density, high-quality two-dimensional electron system in GaAs. At not too high bias voltages V across the PC, the noise temperature is determined by a Joule heat power and is almost independent on the PC resistance that can be associated with a self-heating of the electronic system. This commonly accepted scenario breaks down at increasing V, where we observe extra noise accompanied by a strong decrease of the PC's differential resistance. The spectral density of the extra noise is roughly proportional to the nonlinear current contribution in the PC, ?S?2F*|e?I|~V2, with the effective Fano factor F*<1, indicating that a random scattering process is involved. A small perpendicular magnetic field is found to suppress both ?I and ?S. Our observations are consistent with a concept of a draglike mechanism of the nonlinear transport mediated by electron-electron scattering in the leads of quasiclassical PCs.