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The back scattering cross-section along the axis of a thin walled, circular, tubular cylinder is measured. The cylinder has a length to inner diameter ratio of six to one. The ratio of the length of the cylinder to the wavelength of the incident field varies approximately from 1.8 to 6.3. Near and above the H11 cutoff frequency of the circular waveguide mode of the tubular cylinder, data are distinctly different from previously measured results using cylinders having a length to outer diameter ratio of six to one. The new data agree with theoretical predictions over the entire frequency range. It is concluded that the wall thickness of the cylinder under consideration can be neglected and that the internal (waveguide) modes of a scatterer contribute significantly to its scattering cross-section.