Transparent conductive oxides (TCOs) have emerged as materials for nanoscale electro-optic modulators. The free-carrier-induced epsilon-near-zero effect by applying a gate voltage is capable of achieving ultra-strong electro-absorption (EA) effect. As the EA rate has been theoretically proved to be dependent on the mobility of the TCO gate, we experimentally demonstrated a hybrid silicon-plasmonic EA modulator using high-mobility In2O3 as the gate. With an ultra-compact active region of only 5-μm long, we achieved a small voltage swing Vpp of only 2 V to obtain an EA rate of 1.2 dB / μm, resulting in very high energy efficiency of 110 femto-joule/bit. We also experimentally proved the correlation between the EA rate and the mobility of TCO materials through comparison with indium-tin-oxide gated modulators. In addition, the hybrid EA modulator is capable of covering more than 100-nm optical bandwidth in the telecommunication wavelength window, which is limited by the bandwidth of the grating coupler.