  A zero temperature Anderson-Mott transition driven by spin disorder can be
`tuned' by an applied magnetic field to achieve colossal magnetoconductance.
Usually this is not possible since spin disorder by itself cannot localise a
high density electron system. However, the presence of strong structural
disorder can realise this situation, self consistently generating a disordered
magnetic ground state. We explore such a model, constructed to understand
amorphous GdSi, and highlight the emergence of a spin glass phase,
Anderson-Mott signatures in transport and tunneling spectra, and unusual
magneto-optical conductivity. We solve a disordered strong coupling
fermion-spin-lattice problem essentially exactly on finite systems, and account
for all the qualitative features observed in magnetism, transport, and the
optical spectra in this system.
