We have measured the time taken for a magnetically trapped cloud of cold cesium atoms in the (F =3,mF=-3) ground state to rethermalize from a nonequilibrium spatial and velocity distribution. From these measurements we infer the dependences of the elastic scattering cross section on temperature and magnetic field in the ranges 1 - 30 μΚ and 0.05-2.0 mT, respectively. We determine a lower bound on the magnitude of the (3,-3) + (3,-3) s-wave scattering length of 940a0.
