Here are two pens that are perfectly suited for travel or field notebooks: The Muji thin ballpoint pen and the ohto minimo ballpoint pen:

Both pens are short – about 3.5 inches, and very thin. The Muji Thin pen is a tiny sheathed pen that can expand into a full length pen if needed. The Ohto has a click mechanism with Ohto’s classic side click retract feature. My pens have black ink and write very fine lines:

As far as writing goes, both pens did great, especially when their tiny size is factored in Both pens wrote without skipping and resisted smuging reasonably well. I used a small Fabriano notebook for this comparison.
Here is the Muji Thin Pen up close:

Here’s the Ohto Minimo Ballpoint pen up close:

Comfort-wise, the Muji Thin pen has an edge, because you can adjust its length and angle it against your hand when writing. With both pens, though, the grip is pinched, and neither would be great for prolonged writing. I guess you could wrap some electrical tape around the barrels if you needed to MacGyver them.
Follow up a few weeks later: I carry the Muji Thin pen around and use it. Sometimes the refill hangs out of the pen sheath a bit, because it is held in with a spring. Easily fixable, but notable. The Muji continues to write quite well. Less data on the Ohto, but the refill locks into place when it is clicked, and seems more secure.
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