  We have measured directly the thermal conductance between electrons and
phonons in ultra-thin Hf and Ti films at millikelvin temperatures. The
experimental data indicate that electron-phonon coupling in these films is
significantly suppressed by disorder. The electron cooling time $\tau_\epsilon$
follows the $T^{-4}$-dependence with a record-long value $\tau_\epsilon=25ms$
at $T=0.04K$. The hot-electron detectors of far-infrared radiation, fabricated
from such films, are expected to have a very high sensitivity. The noise
equivalent power of a detector with the area $1\mum^2$ would be
$(2-3)10^{-20}W/Hz^{1/2}$, which is two orders of magnitude smaller than that
of the state-of-the-art bolometers.
