Multiwalled carbon nanotubes (MWNTs) are shown to be ballistic conductors at room temperature, with mean free paths of the order of tens of microns. The electrical transport measurements are performed both in air and in high vacuum in the transmission electron microscope on nanotubes pointing out of a nanotube-containing fiber that contact with a liquid metal. These measurements demonstrate that metallic MWNTs are one dimensional conductor that have quantized conductance nearly 1G0 (~{!V~}(12.9 K~{ and 8~})-1). The intrinsic resistance per unit length is found to be smaller than 100 ~{ and 8~}/~{ and L~}m, indicating a mean free path l> 65 ~{ and L~}m. The nanotube-metal contact resistances are in the range from 0.1 to 1 k~{ and 8 and L~}m. Contact scattering can explain why the measured conductances are about half of the expected theoretical value of 2G0. Current-to-voltage characteristic are in accord with the electronic structure. The nanotubes can survive high current (up to 1 mA, i.e., current density on the order of 109 A/cm2). In situ electron microscopy shows that a relative large fraction of the nanotubes do not conduct (even at high bias), consistent with the existence of semiconducting nanotubes. Discrepancies with other measurements are most likely due to damage caused to the outer layer(s) of the nanotubes during processing. The measured mean free path of clean, undamaged arc-produced MWNTs is several orders of magnitude greater than that for metals, making this perhaps the most significant property of carbon nanotubes.