  A theory of recently discovered anomalous low-field magnetoresistance is
developed for the system of two-dimensional electrons scattered by hard disks
of radius $a,$ randomly distributed with concentration $n.$ For small magnetic
fields the magentoresistance is found to be parabolic and inversely
proportional to the gas parameter, $ \delta \rho_{xx}/\rho \sim - (\omega_c
\tau)^2 / n a^2.$ With increasing field the magnetoresistance becomes linear
$\delta \rho_{xx}/\rho \sim - \omega_c \tau $ in a good agreement with the
experiment and numerical simulations.
