Quantum beats of electron Larmor precession in GaAs wells

We observe quantum beats in the time-resolved photoluminescence of GaAs quantum wells in a magnetic field perpendicular to the growth direction. These beats originate from Larmor precession of electron spins and show a coherence time of 500 ps, which is much longer than the optical dephasing time of 7 ps. The long coherence time allows us to determine the electron Land\'e g factors with an accuracy better than 1%.