Nonmonotonic magnetoresistance of two-dimensional electron systems in the ballistic regime

We report experimental observations of a novel magnetoresistance (MR) behavior of two-dimensional electron systems in perpendicular magnetic field in the ballistic regime for ${k}_{B}T\ensuremath{\tau}/\ensuremath{\hbar}g1$. The MR grows with field and exhibits a maximum at fields $Bg1/\ensuremath{\mu}$ where $\ensuremath{\mu}$ is the electron mobility. As temperature increases, the magnitude of the maximum grows and its position moves to higher fields. This effect is universal: it is observed in various Si- and GaAs-based two-dimensional electron systems. We compared our data with recent theory based on the Kohn anomaly modification in magnetic field and found qualitative similarities and discrepancies.