  We introduce phase space concepts to describe quantum states in a disordered
system. The merits of an inverse participation ratio defined on the basis of
the Husimi function are demonstrated by a numerical study of the Anderson model
in one, two, and three dimensions. Contrary to the inverse participation ratios
in real and momentum space, the corresponding phase space quantity allows for a
distinction between the ballistic, diffusive, and localized regimes on a unique
footing and provides valuable insight into the structure of the eigenstates.
