  Using a three-frequency one-dimensional kicked rotor experimentally realized
with a cold atomic gas, we study the transport properties at the critical point
of the metal-insulator Anderson transition. We accurately measure the
time-evolution of an initially localized wavepacket and show that it displays
at the critical point a scaling invariance characteristic of this second-order
phase transition. The shape of the momentum distribution at the critical point
is found to be in excellent agreement with the analytical form deduced from
self-consistent theory of localization.
