Nuggets are more stick like (so it mirrors the ladder recipe) and it will prevent conflicts with mods that take inspiration from the vanilla recipe to add iron horse armour :)

{
"type": "minecraft:crafting_shaped",
"pattern": [
"# #",
"#i#",
"# #"
],
"key": {
"#": {
"item": "minecraft:iron_nugget"
},
"i": {
"item": "minecraft:iron_ingot"
}
},
"result": {
"item": "quark:iron_ladder",
"count": 1
}
}