3-dimensional particle in a box

If I know how to get a wave function for a 3-dimensional particle in a box problem, what adjustments must I make to solve the same problem for a particle traveling in the +z direction in a tube of infinite length?

Box:
0<x<a
0<y<b
0<z<c

Tube:
0<x<a
0<y<a
0<z<infinity

 

well i think i problem i'm running into is that the wave function for the box isin the form of a sine function Asin((n pi x)/a)sin((n pi y)/b). sin((n pi z)/c). But if i just sub in my infinity then that doesn't really make sense, it just goes to sin(0).