  We measure the relaxation rate $W \equiv T_{1}^{-1}$ of a single electron
spin in a quantum dot at magnetic fields from 7 T down to 1.75 T, much lower
than previously measured. At 1.75 T we find that $T_{1}$ is 170 ms. We find
good agreement between our measurements and theoretical predictions of the
relaxation rate caused by the spin-orbit interaction, demonstrating that
spin-orbit coupling can account for spin relaxation in quantum dots.
