The ratio of the transverse diffusion coefficient to the mobility, D-T/K, has been experimentally determined for Cs+ ion drifting in He gas at E/N (electric field to neutral gas number density ratio) values ranging from 5 to 140 Td. Results at 303 K are reported with an overall accuracy of +/-4%. The D-T/K results were effectively corrected for longitudinal end effects existing in the drift tube. In addition, Monte Carlo simulation (MCS) calculations were made for the mobility and transport coefficients using the Koutselos, Mason, and Viehland (KMV) interaction potential as input. Comparisons of MCS results and available kinetic theory’s Gram-Charlier results with the present and previous experimental data give an indication of the validity and accuracy of the present potential. The theoretical results did not reproduce the experimental curve very well, suggesting that the proposed potential for the Cst-He pair should be modified. Further comparisons of the transverse and longitudinal diffusion coefficients indicate that the Gram-Charlier approach of kinetic theory is quite accurate even for the large ion-neutral mass ratio.