  We theoretically study the dipolar motion of bosonic atoms in a very shallow,
strongly confined 1D optical lattice using the parameters of the recent
experiment [Fertig et al., Phys. Rev. Lett. 94, 220402 (2005)]. We find that,
due to momentum uncertainty, a small, but non-negligible, atom population
occupies the unstable velocity region of the corresponding classical dynamics,
resulting in the observed dissipative atom transport. This population is
generated even in a static vapor, due to quantum fluctuations which are
enhanced by the lattice and the confinement, and is not notably affected by the
motion of atoms or finite temperature.
