  We report new measurements of the drift velocity and longitudinal diffusion
coefficients of electrons in pure xenon gas and in xenon-helium gas mixtures at
1-9 bar and electric field strengths of 50-300 V/cm. In pure xenon we find
excellent agreement with world data at all $E/P$, for both drift velocity and
diffusion coefficients. However, a larger value of the longitudinal diffusion
coefficient than theoretical predictions is found at low $E/P$ in pure xenon,
below the range of reduced fields usually probed by TPC experiments. A similar
effect is observed in xenon-helium gas mixtures at somewhat larger $E/P$. Drift
velocities in xenon-helium mixtures are found to be theoretically well
predicted. Although longitudinal diffusion in xenon-helium mixtures is found to
be larger than anticipated, extrapolation based on the measured longitudinal
diffusion coefficients suggest that the use of helium additives to reduce
transverse diffusion in xenon gas remains a promising prospect.
