  The plateaus observed in about one half of the early X-ray afterglows are the
most puzzling feature in gamma-ray bursts (GRBs) detected by Swift. By
analyzing the temporal and spectral indices of a large X-ray plateau sample, we
find that 55% can be explained by external, forward shock synchrotron emission
produced by a relativistic ejecta coasting in a \rho ~ r^{-2}, wind-like
medium; no energy injection into the shock is needed. After the ejecta collects
enough medium and transitions to the adiabatic, decelerating blastwave phase,
it produces the post-plateau decay. For those bursts consistent with this
model, we find an upper limit for the initial Lorentz factor of the ejecta,
\Gamma_0 \leq 46 (\epsilon_e/0.1)^{-0.24} (\epsilon_B/0.01)^{0.17}; the
isotropic equivalent total ejecta energy is E_{iso} ~ 10^{53}
(\epsilon_e/0.1)^{-1.3} (\epsilon_B/0.01)^{-0.09} (t_b/10^4 s) erg, where
\epsilon_e and \epsilon_B are the fractions of the total energy at the shock
downstream that are carried by electrons and the magnetic field, respectively,
and t_b is the end of the plateau. Our finding supports Wolf-Rayet stars as the
progenitor stars of some GRBs. It raises intriguing questions about the origin
of an intermediate-\Gamma_0 ejecta, which we speculate is connected to the GRB
jet emergence from its host star. For the remaining 45% of the sample, the
post-plateau decline is too rapid to be explained in the coasting-in-wind
model, and energy injection appears to be required.
