  The appearance of a broad peak at $\sim$1550 cm$^{-1}$ (G$^{-}$ peak) in
carbon nanotube resonant Raman scattering spectra has been conventionally
attributed to the presence of metallic nanotubes. Here, we present resonant
Raman measurements on macroscopic nanotube ensembles enriched in armchair
species prepared through density gradient ultracentrifugation. Our data clearly
demonstrate that the broad G$^{-}$ mode is absent for armchair structures and
appears only when the excitation laser is resonant with non-armchair "metals."
Due to the large number ($\sim$10$^{10}$) of nanotubes across several armchair
species probed, our work firmly establishes a general correlation between
G-band lineshape and nanotube structure.
