We consider a superlattice of parallel metal tunnel junctions with a
spatially non-homogeneous probability for electrons to tunnel. In such
structures tunneling can be accompanied by electron scattering that conserves
energy but not momentum. In the special case of a tunneling probability that
varies periodically with period a in the longitudinal direction, i.e.,
perpendicular to the junctions, electron tunneling is accompanied by "umklapp"
scattering, where the longitudinal momentum changes by a multiple of h/a. We
predict that as a result a sequence of metal-insulator transitions can be
induced by an external electric- or magnetic field as the field strength is
increased.Comment: 5 pages, 3 figure
