  We study $\Lambda$-type Electromagnetically Induced Transparency (EIT) on the
Rb D2 transition in a buffer-gas-free thermal vapor cell without
anti-relaxation coating. Experimental data show distinguished features of
velocity-selective optical pumping and one EIT resonance. The Zeeman splitting
of the EIT line in magnetic fields up to 12 Gauss is investigated. One Zeeman
component is free of the first-order shift and its second-order shift agrees
well with theory. The full width at half maximum (FWHM) of this
magnetic-field-insensitive EIT resonance is reduced due to Doppler narrowing,
scales linearly in Rabi frequency over the range studied, and reaches about
100~kHz at the lowest powers. These observations agree with an analytic model
for a Doppler-broadened medium developed in Ref.
\cite{PhysRevA.66.013805,7653385}. Numerical simulation using the Lindblad
equation reveals that the transverse laser intensity distribution and two
$\Lambda$-EIT systems must be included to fully account for the measured line
width and line shape of the signals. Ground-state decoherence, caused by
effects that include residual optical frequency fluctuations, atom-wall and
trace-gas collisions, is discussed.
