  An analytical study of the low-field magnetoresistance of a two-dimensional
electron gas subject to a weak periodic modulation is presented. We assume
small-angle impurity scattering characteristic for high-mobility semiconductor
heterostructures. It is shown that the condition for existence of the strong
low-field magnetoresistance induced by so-called channeled orbits is
$\eta^{3/2}ql\gg 1$, where $\eta$ and $q$ are the strength and the wave vector
of the modulation, and $l$ is the transport mean free path. Under this
condition, the magnetoresistance scales as $\eta^{7/2}$.
