  We study momentum relaxation due to dilute, weak impurities in a strongly
coupled CFT, a truncation of the M2 brane theory. Using the AdS/CFT
correspondence, we compute the relaxation time scale as a function of the
background magnetic field B and charge density \rho. The theory admits two
different types of impurities. We find that for magnetic impurities, momentum
relaxation due to the impurity is suppressed by a background B or \rho. For
electric impurities, due to an underlying instability in the theory towards an
ordered phase, the inverse relaxation time scale increases dramatically near
\sqrt{B^2 + \rho^2/\sigma^2_0} \sim 21 T^2. We compute the Nernst response for
the impure theory, and comment on similarities with recent measurements in
superconductors.
