First make the spiral using the “helix” tool under the “lines” tab.
Then make a circle of the thickness you want, centered on one end of the helix.
Now with the circle selected, use the “sweep line” tool under the “surface modeling” tab and click on the helix.
Set “taper” to zero (or some small number, you can play with the value to your liking).
That should give you what you want.
If the thickness of the spiral is backwards (IOW, thick at the narrow end and narrow at the wide end), then undo it so that you have just your circle and helix lines showing.
Go back to the “lines” tab, and all the way on the right is the “reverse a curve” tool.
Select the helix, click the “reverse a curve” tool, and then click on the helix.
Now go back and sweep the circle as above. When you taper it, it will now taper in the proper direction.
Click thumbnail to see full-size image