  The calculation of the heating rate of cold atoms in vibrating traps requires
a theory that goes beyond the Kubo linear response formulation. If a strong
"quantum chaos" assumption does not hold, the analysis of transitions shows
similarities with a percolation problem in energy space. We show how the
texture and the sparsity of the perturbation matrix, as determined by the
geometry of the system, dictate the result. An improved sparse random matrix
model is introduced: it captures the essential ingredients of the problem, and
leads to a generalized variable range hopping picture.
