  Transport of a Brownian particle moving along the axis of a three-dimensional
asymmetric periodic tube is investigated in the presence of asymmetric unbiased
forces. The reduction of the coordinates may involve not only the appearance of
entropic barrier but also the effective diffusion coefficient. It is found that
in the presence of entropic barrier, the asymmetry of the tube shape and the
asymmetry of the unbiased forces are the two ways of inducing a net current.
The current is a peaked function of temperature which indicates that the
thermal noise may facilitate the transport even in the presence of entropic
barrier. There exists an optimized radius at the bottleneck at which the
current takes its maximum value. Competition between the two opposite driving
factors may induce current reversal.
