  The diffusion quantum Monte Carlo technique is used to solve the many-body
Schroedinger equation fully quantum mechanically and nonperturbatively for
bosonic atomic gases in cigar-shaped confining potentials. By varying the
aspect ratio of the confining potential from 1 (spherical trap) to 10000
(highly elongated trap), we characterize the transition from the
three-dimensional regime to the (quasi-)one-dimensional regime. Our results
confirm that the bosonic gas undergoes ``fermionization'' for large aspect
ratios. Importantly, many-body correlations are included explicitly in our
approach.
