  We have observed a large positive quasi-classical magneto-resistance (MR) in
a high mobility 2D electron gas in AlGaAs/GaAs heterostructure. The
magneto-resistance is non-saturating and increases with magnetic field as
$\rho_{xx}\sim B^{\alpha} (\alpha=0.9-1.2)$. In antidot lattices a
non-monotonic MR is observed. We show that in both cases this MR can be
qualitatively described in terms of the theory recently advanced by Polyakov et
al (PRB, 64, 205306 (2001)). Their prediction is that such behavior as we
observe may be the consequence of a concurrent existence of short and long
range scattering potentials.
