  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_BT\tau/\hbar>1. The MR grows with field and
exhibits a maximum at fields B>1/\mu, where \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.
