Yes that sounds right. The change in wavelength dependence comes from the propagation constant (the argument in the exponential) so the peaks in this exponenti

Yes that sounds right. The change in wavelength dependence comes from the propagation constant (the argument in the exponential) so the peaks in this exponential are spaced differently than in freespace. The propagation constant changes in order to satisfy the boundary conditions (physically you could think of the wave bouncing at an angle off the sides of the can as it propagates down it) or you can think of it as a way to solve the helmholtz equation, while satisfying the need for the tangential electric field to vanish at the radius of the can.