  We observe two-fold shell filling in the spectra of closed one-dimensional
quantum dots formed in single-wall carbon nanotubes. Its signatures include a
bimodal distribution of addition energies, correlations in the excitation
spectra for different electron number, and alternation of the spins of the
added electrons. This provides a contrast with quantum dots in higher
dimensions, where such spin pairing is absent. We also see indications of an
additional fourfold periodicity indicative of K-K' subband shells. Our results
suggest that the absence of shell filling in most isolated nanotube dots
results from disorder or nonuniformity.
