Is Planck length the smallest unit of matter?

What is said upstairs is not accurate. Quarks are model particles that make up hadrons such as neutrons and protons. Note that no one can really see it. Theoretically, quarks cannot appear alone. This is called "color confinement".

But even if quarks do exist, they are only one of the smallest particles that make up matter, and they are not complete!

For example, electrons! ! So far, the experiment has achieved the order of 10- 19 m, but electrons still can't reflect any structure, so they are considered as one of the most basic particles. There are also particles in various cosmic rays: neutrinos, gluons, W+, Z particles and so on. At present, they are obviously the most basic particles.

In addition, Planck constant is only a sign reflecting the difference between micro-scale and classical macro-scale! Its existence only expresses the function of quantum mechanics, and the general function scale of quantum theory is atomic level, that is, below the order of 10 (- 10) m, and the elementary particles you mentioned are much smaller than this value. For example, the nucleus is already in the order of 10 (- 15) m ... so the scale marked by Planck constant is much larger than that of elementary particles.