1. The problem statement, all variables and given/known data 2. Relevant equations None. 3. The attempt at a solution I understand the basic concept of Matthiessen's rule that the contribution to electronic resistivity / conductivity is split into two independant componants, resistivity due to impurities and resistivity due to the lattice. But I am unsure how this relates to if an electric field is applied. Also we have been taught it only applies if independant from the wavevector, could someone perhaps emphasise this a little? Please could someone give me a point in the right direction.