  We report a theoretical investigation on conductance fluctuation of
mesoscopic systems. Extensive numerical simulations on quasi-one dimensional,
two dimensional, and quantum dot systems with different symmetries (COE, CUE,
and CSE) indicate that the conductance fluctuation can reach a new universal
value in the crossover regime for systems with CUE and CSE symmetries. The
conductance fluctuation and higher order moments vs average conductance were
found to be universal functions from diffusive to localized regimes that depend
only on the dimensionality and symmetry. The numerical solution of DMPK
equation agrees with our result in quasi-one dimension. Our numerical results
in two dimensions suggest that this new universal conductance fluctuation is
related to the metal-insulator transition.
