Actually this all gets into some rather boring stuff but it goes a little like this. All materials (including copper) have a resistivity. That resistivity is just a base number not to be confused with resistance as measured in ohms. However, knowing the resistivity for a given wire alloy and the size (diameter or cross sectional area) of the wire with the length we can calculate the resistance in Ohms. Resistivity is stated in a unit called the Rho.
Things look like this:
Resistance (Ohms) = Rho * Length / Area
Now the resistivity or Rho of copper (as twisted spark
points out) is about 17 nano Rho or .000000017 Rho which is a pretty damn small number. Knowing the Rho of the material in a conductor we can calculate the resistance.
If that is not boring enough, we get into connectors placed in the line. Each connector meaning every single little pin and socket along the way add resistance to the line. All connectors have specifications that state their resistance. This is all the weird stuff that is considered when designing and building electrical/electronic circuits. The resistance accross most connectors is stated generally in milli (10^-3) Ohms.
Looking at a circuit and the current flow in a circuit we can calculate the voltage drop due to resistance pretty easily. This is commonly called an IR drop or actually it is an I * R drop because if we multiply the Current (I) * the Resistance (R) we get the Voltage drop.
The point here is that in circuits like we are looking at this drop is negligible. This goes back to what I posted earlier. The drop or loss even though it exist will never come in light of the design application.
Now what pisses me off (you were wondering where this was going right?) is how as I mentioned some PSU makers hyping the non-modular concept used this very basic law of physics to discredit the modular PSU design. They were quick to0 stress the connectors caused loss but failed miserably to actually tell the truth. The fact is a few connectors will not matter.
The bulk of the PSU market is not home enthusiast. The bulk of the market is everyday business machines. The market for higher end PSUs is actually pretty small. Thus it has become extremely competative and just about every conceivable gimmic and ploy is used in marketing. That pisses me off.
<EDIT> Oh yeah, to further confuse things the Rho of any alloy changes with temperature so therefore the actual resistance changes with temperature. However, fow all practical purposes, not enough to matter.