Petrographic studies of peridotitic xenoliths entrained in late Quaternary basalts from beneath the southern Sierra Nevada have revealed the presence of accessory sulfide minerals along grain boundaries and fractures. Equilibration temperatures from the xenoliths are sufficiently high that the molten sulfides coexist with the basaltic melt. Sulfides are extremely conductive relative to the solid matrix or the basaltic melt, so a small fraction can increase the bulk conductivity of the mantle appreciably. Previous estimates of 2--5% partial melt from magnetotelluric measurements can be plausibly reduced to less than 1%. Such low melt percentages have longer residence times in the mantle and are more consistent with the volumetrically minor late Quaternary basalt flows and the primitive basalt compositions. ¿ 2000 American Geophysical Union