  Nonequilibrium transport measurements in mesoscopic quasi-ballistic 2D
electron systems show an enhancement in the differential conductance around the
Fermi energy. At very low temperatures, such a zero-bias anomaly splits,
leading to a suppression of linear transport at low energies. We also observed
a scaling of the nonequilibrium characteristics at low energies which resembles
electron scattering by two-state systems, addressed in the framework of
two-channel Kondo model. Detailed sample-to-sample reproducibility indicates an
intrinsic phenomenon in unconfined 2D systems in the low electron-density
regime.
