  We provide evidence that as a general rule Anderson localization effects
become weaker as the degree of differentiability of the disordered potential
increases. In one dimension a band of metallic states exists provided that the
disordered potential is sufficiently correlated and has some minimum degree of
differentiability. Several examples are studied in detail. In agreement with
the one parameter scaling theory the motion in the metallic region is ballistic
if the spectral density is smooth. Finally, we study the most promising
settings to observe these results in the context of cold atoms.
