As a general rule, low-cost alligator-style leads like those pictured below are examples of terrible manufacturing quality, which has been observed to grow progressively worse as both the physical size and cost of the product decrease.
The common problem with these products is a poor connection between the wire lead and the alligator clips. The best one can expect of these products is that a stripped end of wire will be neatly folded back over the insulation and badly crimped into place, like this:
But you’re also likely to get a stripped end that looks like medusa on a bad hair day and a crimp scarcely worthy of the name, like this:
The result is that if the clips and wires don’t promptly separate, they’ll have a high resistance and/or intermittent character that only becomes worse with time as the wire and clip corrode. New from the package, a four-wire resistance measurement through the lead pictured showed a resistance of about an ohm, which is about 20 times what one would expect based on length and the claimed wire size. Others in the pack had similar measurements, which varied significantly when wire was moved relative to clip, indicating the beginnings of an intermittent character.
It should be understood that this level of quality is typical for low-cost products of this type, and one should not expect better unless paying at least a few dollars each.
This said, a soldered connection and a decent crimp can make products like this usable. It’s best to do it immediately after you take them out of the manufacturer’s packaging, before the wire corrodes and becomes difficult to solder, and before a bad lead causes you problems. (Hint: solder first, then re-crimp wire in place to minimize melting of insulation.)
For those interested in building their own to start with rather than re-working a poorly-assembled product, a silicone-insulated test lead wire is preferred by many for it’s superior handling qualities, and clips can be found here.