We use uncoupled Co/Cu/Co and Co/Cu/Co/Cu spin-valve structures with a Ru barrier shifted through the top Co and Cu layer, respectively, to measure the longest of the electron mean free paths in Co and Cu as originally suggested by Parkin. From semiclassical transport calculations and careful analysis of the magnetoresistance data we conclude that the exponential behavior of ⌬G is uniquely related to the longest of the Co and Cu mean free paths under the condition of effective spin-dependent

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... tering at the interfaces or in the bulk of the Co. In this regime we have compared long in Co and Cu with bulk conductivities ͑ϰ short ϩ long ͒, yielding no strong evidence for bulk spin-dependent scattering in Co.