  We present angle and frequency resolved optical extinction measurements to
determine the dispersion relation of plasmon modes on Ag and Au nanoparticle
chains with pitches down to 75 nm. The large splitting between transverse and
longitudinal modes and the band curvature are inconsistent with reported
electrostatic near-field models, and confirm that far-field retarded
interactions are important, even for $\lambda/5$-sized structures. The data
imply that lower propagation losses, larger signal bandwidth and larger maximum
group velocity then expected can be achieved for wave vectors below the light
line. We conclude that for the design of optical nanocircuits coherent
far-field couplings across the entire circuit need to be considered, even at
subwavelength feature sizes.
