  We investigate the monotonic growth of longitudinal interlayer
magnetoresistance $\bar{R}_{zz}(B_z) $, analytically and numerically in the
self-consistent Born approximation. We show that in a weak magnetic field the
monotonic part of $\bar{R}_{zz}(B_z)$ is almost constant and starts to grow
only above the crossover field $B_{c}$, when the Landau levels (LL) become
isolated, i.e. when the LL separation becomes greater than the LL broadening.
In higher field $B_{z}>>B_{c}$, $\bar{R}_{zz}(B_{z}) \propto B_{z}^{1/2}$ in
agreement with previous works.
