  Using spectroscopic data from the Deep Extragalactic Evolutionary Probe
(DEEP) Groth Strip survey (DGSS), we analyze the gas-phase oxygen abundances in
the warm ionized medium for 64 emission-line field galaxies in the redshift
range 0.26<z<0.82. Oxygen abundances relative to hydrogen range between
8.4<12+log(O/H)<9.0 with typical internal plus systematic measurement
uncertainties of 0.17 dex. The 64 DGSS galaxies collectively exhibit an
increase in metallicity with B-band luminosity. DGSS galaxies in the highest
redshift bin (z=0.6-0.82) are brighter, on average, by ~1 mag at fixed
metallicity compared to the lowest DGSS redshift bin (z=0.26-0.40) and brighter
by up to ~2.4 mag compared to local (z<0.1) emission-line field galaxies.
Alternatively, DGSS galaxies in the highest redshift bin (z=0.6-0.82) are, on
average, 40% (0.15 dex) more metal-poor at fixed luminosity compared to local
(z<0.1) emission-line field galaxies. For 0.6<z<0.8 galaxies, the offset from
the local L-Z relation is greatest for objects at the low-luminosity (M_B>-19)
end of the sample and vanishingly small for objects at the high-luminosity end
of the sample (M_B ~ -22). Simple galaxy evolution models can produce
reasonable agreement with observations for low-mass galaxies when least two of
the following are true: 1) low-mass galaxies have lower effective chemical
yields than massive galaxies, 2) low-mass galaxies assemble on longer
timescales than massive galaxies, 3) low-mass galaxies began the assembly
process at a later epoch than massive galaxies. (abridged)
