  We report on high-temperature electron spin resonance studies of A-site
ordered YBaMn2O6 and disordered Y0.5Ba0.5MnO3. In the disordered sample we find
that the linewidth is governed by spin-spin relaxation processes as in many
other manganite systems. In contrast we find a Korringa-like spin relaxation
with a slope of about 1 Oe/K above the charge-ordering transition and extending
up to 930K in the metal ordered YBaMn2O6 samples. A Korringa-law is a clear
feature of a truly metallic state, pointing to an unconventional
high-temperature phase of these ordered manganites. In agreement with the ESR
intensity this suggests that in the metallic high-temperature phase the ESR
signal stems from Mn^4+ core spins which relax via the quasi-delocalized e_g
electrons.
