  The maximum bit-rate of a slab waveguide is ultimately determined by the
waveguide dispersion. We show that while the maximum bit rate in a waveguide is
inversely proportional to the waveguide's width, bit rate per unit width (i.e.,
spatial capacity) decreases, and in the limit of a zero-width waveguide it
converges to a value, which is independent of the waveguide's refractive
indices. This value is qualitatively equivalent to the transmission rate per
unit of width in free space. We also show that in a 3D waveguide (e.g.,
fibers), unlike free space, the spatial capacity vanishes in the same limit.
