Abstract
A method is developed to calculate the localisation length in disordered systems. It can be considered to be a generalisation of the method of Herbert, Jones and Thouless (1971-2) for one-dimensional systems, to wires and nu -dimensional lattices. The localisation length can be calculated under some assumptions when the disorder has a Lorentzian probability distribution. No localisation transition is found, that is the localisation length is finite for arbitrarily small non-zero values of the disorder. For other distributions a perturbation expansion is developed, which is valid for large values of disorder and small energies. The dominant term and the first-order correction are calculated.