  We have measured the temperature- and frequency dependence of the microwave
and millimeter wave conductivity $\sigma_1(T,\omega)$ along both the ladder
(c-axis) and the leg (a-axis) directions in Sr$_{14}$Cu$_{24}$O$_{41}$. Below a
temperature $T^*$($\sim$170 K), we observed a stronger frequency dependence in
$\sigma_1^c(T,\omega)$ than that in $\sigma_1^a(T,\omega)$, forming a small
resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear
dc conduction along the c-axis at rather low electric fields below $T^*$. These
results suggest some collective excitation contributes to the c-axis charge
dynamics of the slightly hole-doped ladders of Sr$_{14}$Cu$_{24}$O$_{41}$ below
$T^*$.
