The guitar music in the westerns you mentioned is a combination of Mexican music and bluegrass music, a branch of American country music. Because Mexico borders California, California music is greatly influenced by Mexican music. Mexico is a traditional Spanish colony, so Spanish guitar has an important influence on Mexican music. The fast string sweeping you mentioned is actually an important skill in flamenco guitar music. You can find this kind of music in bluegrass music, and the representative artists are Ricky Scaggs and Doyle Lawson.