I'd expect most of the resistance to be in the bond wire(s),
you could probably figure rough pin material resistance as
dimensional copper with a pair of calipers. Crack a package
and see what the wire bond looks like, for length and diameter,
and there's tables for that.
They are copper plated with tin. Tin gets covered with an oxide layer but this oxide is soluble in the solder flux.
Tin alloys well with copper and the connection is ohmic.
The lead resistance will be in the microohm range (10-100 microohm)
This is close to the value of a well soldered PCB connection.
1. It is not tin because tin cannot stand soldering temp
2. It is not SS because stainless steel cannot take solder
3. Is it steel? you can check with a magnet.