  We discuss the semi-classical kinetic equation in the clean limit, with the
presence of Berry curvatures and magnetic field B, with the aim of applying to
Weyl semi-metals. Special attention is given to the conservation laws for the
collision integrals. It is found that the magneto-resistance second order in B
is in general negative, with or without Weyl points, though in the later case
it is in general much smaller.
