This paper reports broadband THz free-space transmission measurements and modeling of indium-tin-oxide (ITO) thin films on p-doped Si substrates. Two such samples having ITO thickness of 50 and 100 nm, and DC sheet conductance 260 and 56 Ω/sq, respectively, were characterized between 0.2 and 1.2 THz using a frequency-domain spectrometer. The 50-nm-film sample displayed very flat transmittance over the 1-THz bandwidth, suggesting it is close to the critical THz sheet conductance that suppresses multi-pass interference and thereby achieves THz passive equalization. This is consistent with a simple transmission-line (TEM wave) model of the propagation. Also, the value of THz sheet conductance that best fits the transmittance data is roughly 50% higher than the dc value for both samples, suggesting that the ac conductivity is non-Drudian.