We study the effect of magnetic scattering on transport in a system with
strong structural disorder, using exact finite size calculation of the low
frequency optical conductivity. At weak electron-spin coupling spin disorder
leads to a decrease in resistivity by weakening the quantum interference
precursors to Anderson localisation. However, at strong electron-spin coupling,
the double exchange limit, magnetic scattering increases the effective
disorder, sharply increasing the resistivity. We illustrate the several unusual
transport regimes in this strong disorder problem, identify a re-entrant
insulator-metal-insulator transition, and map out the phase diagram at a
generic electron density.Comment: 7 pages, EPL style, with 4 embedded figs. To appear in Europhys. Let
