  We determine the quantum scattering time tau_q in six graphene samples with
mobility of 4,400 < mu < 17,000 cm^2/Vs over a wide range of carrier density
(1.2 < n < 6x10^{12}/cm^2). tau_q derived from Shubnikov-de Haas oscillations
ranges ~25-74 fs, corresponding to a single-particle level broadening of 4.5-13
meV. The ratio of the transport to quantum scattering time tau_t/tau_q spans
1.5-5.1 in these samples, which can be quantitatively understood combining
scattering from short-ranged centers and charged impurities located within 2 nm
of the graphene sheet. Our results suggest that charges residing on the SiO_2
surface play a dominant role in limiting carrier mobility in current samples.
