  We show how the collapse of an energy scale in a quantum critical metal can
lead to physics beyond the weak-field limit usually used to compute transport
quantities. For a density-wave transition we show that the presence of a finite
magnetic field at the critical point leads to discontinuities in the transport
coefficients as temperature tends to zero. The origin of these discontinuities
lies in the breakdown of the weak field Jones-Zener expansion which has
previously been used to argue that magneto-transport coefficients are
continuous at simple quantum critical points. The presence of potential
scattering and magnetic breakdown rounds the discontinuities over a window
determined by tau Delta < 1 where Delta is the order parameter and tau is the
quasiparticle elastic lifetime.
