  We study the transport of ultra cold atoms in a tight optical lattice. For
identical fermions the system is insulating under an external force while for
bosonic atoms it is conducting. This reflects the different collisional
properties of the particles and reveals the role of inter-particle collisions
in establishing a macroscopic transport in a perfectly periodic potential. Also
in the case of fermions we can induce a transport by creating a collisional
regime through the addition of bosons. We investigate the transport as a
function of the collisional rate and we observe a transition from a regime in
which the mobility increases with increasing collisional rate to one in which
it decreases. We compare our data with a theoretical model for electron
transport in solids introduced by Esaki and Tsu.
