Abstract
Scaling theories of localisation in disordered materials predict that the conductance and localisation lengths respectively vary as mod E-Ec mod above and below the mobility edge. On the other hand some experiments have observed conductances varying as mod E-Ec mod 12/. In this paper it is shown that for systems having highly anisotropic effective mass tensors, localisation occurs for small values or disorder, which enables an analytic theory to be developed. This anisotropic theory gives an exponent of 1/2. The theory is applicable to n-type silicon provided that inter-valley scattering can be neglected, a result which accords with recent observations. The theory also shows in a natural way how the 1D and 2D limits of localisation theory can be taken by increasing the effective mass in one or two directions. The well known result that all states are localised in 2D is reproduced and can be ascribed to underlying symmetries in the system.