  The resistivity change due to electron spin resonance (ESR) absorption is
investigated in a high-mobility two-dimensional electron system formed in a
Si/SiGe heterostructure. Results for a specific Landau level configuration
demonstrate that the primary cause of the ESR signal is a reduction of the spin
polarization, not the effect of electron heating. The longitudinal spin
relaxation time T_1 is obtained to be of the order of 1 ms in an in-plane
magnetic field of 3.55 T. The suppression of the effect of the Rashba fields
due to high-frequency spin precession explains the very long T_1.
