  We have experimentally demonstrated that magnetic-field-insensitive
coherent-population-trapping (CPT) resonances are generated between the ground
hyperfine levels on the $D_1$ line of $^{133}$Cs using a two-photon $\Lambda$
scheme excited by lin || lin polarizations. The frequency shift of the CPT
resonance is 0.04 Hz for the deviation of 1 $\mu$T at a "magic" magnetic field
of 139 $\mu$T and is 50 times smaller than that of the conventional clock
transition at a typical bias magnetic field for the clock operation. The
amplitude of the CPT spectrum excited by lin || lin polarizations is enhanced
as the excitation intensity increases and is well explained by the three-level
model without trap levels. Thus, the CPT resonance on the $D_1$ line of
$^{133}$Cs atom excited by a simple lin || lin scheme will be one of the best
candidates for frequency reference of miniature atomic clocks.
